As to the load sensing control technique for controlling the displacement of a hydraulic pump so that a differential pressure between a delivery pressure of the hydraulic pump and a maximum load pressure among a plurality of actuators is maintained at a setting value, there are known a pump displacement control system disclosed in JP, A, 5-99126 and a hydraulic drive system disclosed in JP, A, 60-11706.
The pump displacement control system disclosed in JP, A, 5-99126 comprises a servo piston for tilting a swash plate of a variable displacement hydraulic pump, and a tilting control unit for supplying a pump delivery pressure to the servo piston in accordance with a differential pressure .DELTA.PLS between a delivery pressure Ps of the hydraulic pump and a load pressure PLS of an actuator driven by the hydraulic pump so as to maintain the differential pressure .DELTA.PLS at a setting value .DELTA.PLSref, thereby controlling the pump displacement. The disclosed pump displacement control system further comprises a fixed displacement hydraulic pump driven by an engine along with the variable displacement hydraulic pump, a throttle disposed in a delivery line of the fixed displacement hydraulic pump, and setting modifying means for modifying the setting value .DELTA.PLSref of the tilting control unit in accordance with a differential pressure .DELTA.Pp across the throttle. The setting value .DELTA.PLSref of the tilting control unit is modified by detecting an engine rotational speed based on change in the differential pressure across the throttle disposed in the delivery line of the fixed displacement hydraulic pump.
The hydraulic drive system disclosed in JP, A, 60-11706 comprises a variable displacement hydraulic pump, a plurality of actuators driven by a hydraulic fluid delivered from the hydraulic pump, a plurality of flow control valves for controlling flow rates of the hydraulic fluid supplied from the hydraulic pump to the plurality of actuators, a plurality of pressure compensating valves controlling differential pressures across the plurality of flow control valves to become equal to each other, and a pump displacement control unit for controlling the displacement of the hydraulic pump so that a differential pressure .DELTA.PLS between a delivery pressure Ps of the hydraulic pump and a maximum load pressure PLS among the plurality of actuators is maintained at a setting value .DELTA.PLSref. The pressure compensating valves are installed upstream of the flow control valves, respectively. Each pressure compensating valve is arranged to receive the differential pressure across the flow control valve acting in the valve-closing direction and the differential pressure .DELTA.PLS between the delivery pressure Ps of the hydraulic pump and the maximum load pressure PLS among the plurality of actuators in the valve-opening direction, for thereby controlling the differential pressure across the flow control valve with the differential pressure .DELTA.PLS as a target differential pressure for pressure compensation. As a result, the differential pressures across the plurality of flow control valves are controlled to become equal to each other.